Method for the controlling of certain second phases in aluminum nitride

ABSTRACT

Electronic packages made with a high area percent coverage of blanket metal may be prone to certain kinds of ceramic defects. In aluminum nitride, these defects may be related to decomposition of the liquid sintering aid. In this experiment, unique additions to the metallization prevented the formation of certain ceramic defects. Our approach involves a unique composition used in an existing process.

This is a continuation divisional of application(s) Ser. No. 08/887,375filed on Jul. 2, 1997, now U.S. Pat. 6,004,624 .

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to densified ceramics bodies forelectronic components and, in particular, to a densified ceramic bodywhich is relatively free of surface defects.

2. Description of Related Art

Aluminum nitride (AlN) is of great interest as a material for electronicpackages because of its high thermal conductivity and close match inthermal expansion to silicon. AlN can be densified at relatively lowtemperatures (1500-1700° C.) by additions of variouslow-melting-temperature compounds. Because of its high equilibrium vaporpressure, densification of the AlN ceramic can only be accomplished byhot pressing or liquid phase sintering.

Electronic packages using AlN as a dielectric are typically processedfrom ceramic tape or greensheets. Greensheets consist of aluminumnitride powder, sintering aids, and an organic binder. The electricalconductor in the package is formulated into a paste made of refractorymetal powder, ceramic additives, and organic binder. These twocomponents are used in the process illustrated in Figs. 1A-1D. The paste12, 14 is screened essentially in a blanket pattern on the top sheet 16of the greensheet stack 10, as illustrated in FIG. 1A, where up to 80%of the surface of the greensheet may be covered by the paste. Often, twodifferent pastes are deposited, one on top of the other. One paste maybe added to provide adhesion to the ceramic and another to offer asurface which has sufficient exposed metal to allow plating. Greensheetsare provided with through holes called vias which are also filled withconductive paste. The sheets 16 are then laminated by applyingsufficient heat and pressure and the result is illustrated in FIG. 1B.The laminated stack 18 is then heated to remove the organic binder fromthe paste 12,14 and from the greensheet 16. Further heating sinters thepowders of both the paste and the greensheet. The resulting stack isillustrated in FIG. 1C. In the sinter cycle, the sintering aids form aliquid which further aids densification of the ceramic body 20. If anycomponent of the liquid sintering aids has a high vapor pressure, theliquid phase will tend to wick to the surface(s) of the part. Thesurface of the part may then tend to have an accumulation of sinteringaid byproducts. The byproducts may be in the form of particles 22 whichprotrude above the surface as illustrated in FIG. 1D. The protrudingparticles 22 are especially undesirable on the surface of an electronicchip carrier because the particles present a debris hazard in clean roomoperations and may also damage a chip during the chip attach process.The chip damage problem is especially exaggerated when the metal coversa high area percent of the ceramic.

Processing of electronic packages is greatly aided by use of amultilayer ceramic (MLC) technology, in which greensheets are cast usingAlN and metallization is applied in the form of thixotropic ink orpaste. The ink typically contains a solvent, a metal powder, a binder,and ceramic additives. The particle size of the metal powder and thevolume fraction of the ceramic additives are chosen in order to assure ashrinkage match between the metallization and the ceramic. The ceramicadditives are typically similar to that of the powder mixture in thegreensheet. In cases where there is a surface metal feature to which IOdevices (wire bonds, pins, tape automated bonding (TAB), etc.) will beattached, mechanical adhesion between the metal and the ceramic isrequired. Here, ceramic additives to the ink are also of great value, inthat they may provide an interlocking feature between the ceramic andmetal.

In some electronic packaging applications, however, there is a need forcoverage of much of the surface of the ceramic with metallization. It ispossible for 70% or more of the ceramic surface to be taken up by metalfeatures. The specifications for the surfaces of these packages havevery stringent limits on the height of bumps or bulges in the ceramic,in order to prevent damage to die which are later attached to thepackages. There is also a strong desire to limit the potential for theceramic to be a source of particulate contamination in a clean roomenvironment.

In a package with high surface metal loading, however, a problem withthe sintering aid may occur. The sintering aid may have a tendency tomove toward the surface during the sinter cycle. Since a highly loadedsystem will have few areas which are not covered by metallization, theliquid phase may accumulate in these areas. Furthermore, the liquidphase is likely to decompose into phases which are of a high hardness.This decomposed phase may cause an electronic package to violatespecifications for surface defect height or to cause concerns aboutclean room contamination. In some extreme cases, the surface secondphase may cause bulges in the metallization.

Accordingly, it is highly desirable to develop aluminum nitride bodieswith minimal surface defects.

Bearing in mind the problems and deficiencies of the prior art, it istherefore an object of the present invention to provide a ceramicsurface for an electronic component whereby the surface is substantiallyfree of surface defects.

It is another object of the present invention to provide an aluminumnitride body with minimal surface defects.

A further object of the invention is to provide a method of producing aceramic body which is substantially free of surface defects.

It is yet another object of the present invention to provide a method ofproducing an aluminum nitride body with minimal surface defects.

Still other objects and advantages of the invention will in part beobvious and will in part be apparent from the specification.

SUMMARY OF THE INVENTION

The above and other objects, which will be apparent to those skilled inthe art, are achieved by the present invention which in a first aspectrelates to a ceramic substrate with a depletion zone. The sinteredaluminum nitride body comprising a substrate of aluminum nitride havinga microstructure containing a compound of aluminum oxide and calciumoxide. The substrate has on at least a portion of a surface thereof alayer comprising a sintered mixture of a refractory metal, a binder andtricalcium aluminate, such that a yttrium aluminate compound precipitatein the substrate is uniformly throughout the microstructure of thesubstrate except in the portion of the substrate adjacent to the layerwherein the microstructure of the substrate is depleted of the compound.

In another aspect, the present invention relates to a method of makingthe substrate with a depletion zone. The method for reducing formationof particles in surface and subsurface microstructures of aluminumnitride bodies comprises the steps of: a) providing a greensheetcomprising aluminum nitride powder, a sintering aid and a binder; b)providing a paste comprising a refractory metal powder, a binder and acompound selected from the group consisting of tricalcium aluminate orother phases in the calcia-alumina system; c) applying the paste to atleast a portion of a surface of the greensheet; and d) heating thegreensheet and paste for a time and temperature sufficient to sinter thegreensheet and paste.

In another aspect, the present invention relates to a paste compositionfor application to a greensheet for aluminum nitride bodies containingcompounds from the calcia-alumina system. The paste application reducesformation of particles in surface and subsurface microstructures thereofcomprising a refractory metal powder, a binder and a compound selectedfrom the group consisting of tricalcium aluminate or the phases of thecalcia-alumina system.

The preferred embodiments are as follows. Where tricalcium aluminate isthe additive, it is added in a range of from about 1 to about 35 percentby volume of the paste, with 22 percent by volume being preferred.

BRIEF DESCRIPTION OF THE DRAWINGS

The features of the invention believed to be novel and the elementscharacteristic of the invention are set forth with particularity in theappended claims. The figures are for illustration purposes only and arenot drawn to scale. The invention itself, however, both as toorganization and method of operation, may best be understood byreference to the detailed description which follows taken in conjunctionwith the accompanying drawings in which:

FIGS. 1A-D1A, B, C, D illustrate the prior art metallization process andthe resulting ceramic with second phase particles in the surface of theceramic.

FIG. 2A is an illustration of a stack of greensheets screened with firstand second layers of metal paste.

FIG. 2B is an illustration of the screened stack of FIG. 1 afterlamination.

FIG. 2C is an illustration of the laminated stack of FIG. 2 after binderburnoff and sintering.

FIG. 2D is a greatly enlarged illustration of the circled area of FIG.2C illustrating a ceramic body with a depleted zone which is free ofsecond phase particles.

DESCRIPTION OF THE PREFERRED EMBODIMENT(S)

In describing the preferred embodiment of the present invention,reference will be made herein to FIGS. 2A-D of the drawings in whichlike numerals refer to like features of the invention. Features of theinvention are not necessarily shown to scale in the drawings.

The metal paste of the present invention comprises a refractory metalpowder such as tungsten, a binder such as ethyl cellulose, and sinteringaid compounds.

The addition of sintering aid compounds (to be described more fullybelow) to surface metallization paste has been found to be effective inovercoming the surface defect problems described above. These additiveshave been determined to help prevent the formation of surface defects bythe creation of a depletion zone, near the surface of the ceramic, whichis free of precipitated second phase particles. The additive compoundsare of the calcia-alumina system.

Various sintering aids and compounds were added to metallization pastes,such as tungsten pastes, at various volume percents. Total ceramicadditives may range from 5 to 65 volume percent of the non-organicportion of the paste, with 55 volume percent being preferred. Theseadditives included aluminum nitride (AlN), and ceramic sintering aidcompounds such as aluminum oxide (Al₂O₃), tricalcium aluminate or C3A(3CaO*Al₂O₃), a calcium aluminoborate glass and compounds in the calciumalumina system including CaO, Ca₃O, Ca₃Al₂O₆, Ca₁₂Al₁₄ O₃₃, CaAl₂O₄,CaAl₄O₇, CaAl₁₂O₁₉, Al₂O₃. The composition of calcium aluminoborate isdisclosed in U.S. Pat. No. 5,482,903 which is hereby incorporated byreference. Where tricalcium aluminate is the additive, it is added in arange of from about 1 to about 35 percent by volume, with 22 percent byvolume being preferred. Aluminum nitride powder may also be added to thepaste in addition to the ceramic sintering aids in an amount from about1 to about 50 volume percent of the paste.

The method for reducing formation of particles in surface and subsurfacemicrostructures of aluminum nitride bodies is illustrated in FIG. 2A-Dand comprises the steps of providing a greensheet comprising aluminumnitride powder, a sintering aid and a binder; providing a pastecomprising a refractory metal powder, a binder and a compound selectedfrom the group consisting of tricalcium aluminate and aluminum nitridepowder; applying the paste to at least a portion of a surface of thegreensheet; heating the greensheet and paste for a time and temperaturesufficient to sinter the greensheet and paste.

In the first step illustrated in FIG. 2A, first and second layers 12,14of metal paste are applied to green sheets 16 which are placed one ontop of another in the stack 10. Next, as illustrated in FIG. 2B, alamination process is performed and a laminated stack 18 is produced.The sintering step follows and produces the densified ceramic 20 of FIG.2C. FIG. 2D illustrates a greatly enlarged encircled section of FIG. 2Cwith a metalized 24 layer and a depletion zone 26.

Metal paste may be applied in one or two layers in accordance with thepresent invention. Where the paste is applied in only one layer thepaste may comprise aluminum nitride and tricalcium aluminate in a rangefrom about 1 to 35 percent by volume.

Where a two layer method is used, the second layer may have a greaterconcentration of tungsten. The first layer may comprise tricalciumaluminate in a range of from about 1 to about 35 percent by volume ofthe paste. The second layer may comprise aluminum nitride or a compoundwhich does not react with the ceramic, or no additive at all (puretungsten).

The product produced by the above methods has been found to have aunique microstructure in AlN ceramic containing yttrium aluminates,whereby there is localized control of microstructure and phases whichresult in a zone of from 10 microns to 200 microns deep into the partunderneath surface screened or I/O metallization. The depleted zone issubstantially free of precipitated yttrium aluminates but may contain awetted phase.

EXAMPLE

In tests, the results of which are described in Table 1, pastes wereblanket screened providing at least 80% coverage of the greensheet. Themetalization was deposited in either one or two layers. The screenedsheets were stacked and laminated at 6000 psi at 75° C. for 5 minutesand then the laminated parts were sintered at 1600° C. for 24 hours.

The parts were then examined in cross section for evidence of adepletion zone. Four pieces were mounted and a photograph atapproximately 200× magnification was taken in a scanning electronmicroscope (SEM) under conditions which would highlight the yttriumaluminate particles. On each photo, the width of the depletion zone oneach side was measured.

It is important to note that the aluminum nitride ceramic containssintering aids of approximately 2% by volume calcium aluminoborate (CAB)glass and approximately 1% by volume Y₂O₃. Also, other tests yieldedsimilar results when only one paste layer with similar components wasscreened.

TABLE 1 Minimum Average Maximum First Layer zone zone zone Test PasteSecond Layer width, width width, No. Additives* Paste Additives micronsmicrons microns 1 55% AlN 100% tungsten 0 0 0 2 28% AlN, no second layer24 32 41 7% C3A 3 21% AlN, no second layer 53 69 82 14% C3A 4 44% AlN,no second layer 59 61 65 11% C3A 5 33% AlN, no second layer 106 115 12322% C3A 6 28% AlN, 100% tungsten 24 32 47 7% C3A 7 21% AlN, 100%tungsten 12 57 88 14% C3A 8 44% AlN, 100% tungsten 24 41 82 11% C3A 933% AlN, 100% tungsten 59 115 206 22% C3A *C3A is an abbreviation fortricalcium aluminate or 3CaO*Al₂O₃.

The test results illustrate that it is possible to control themicrostructure of the ceramic directly under the surface metalization byunique additions to the metallization paste layer. Additions oftricalcium aluminate or similar compounds of the calcia-alumina systemare shown to greatly reduce the amount of sintering aid byproduct on thesurface after sintering. Typical manufacturing of ceramic packages withblanket metallization requires plateability and a high degree ofadhesion to the ceramic. The approach of the present invention addressesthese concerns.

It should also be noted that the present invention has a uniquefingerprint, a zone in the ceramic which is free of second phaseparticles. The zone starts at the metallization and extends down intothe ceramic for some 5 to 100 microns in depth.

While the present invention has been particularly described, inconjunction with a specific preferred embodiment, it is evident thatmany alternatives, modifications and variations will be apparent tothose skilled in the art in light of the foregoing description. It istherefore contemplated that the appended claims will embrace any suchalternatives, modifications and variations as falling within the truescope and spirit of the present invention.

Thus, having described the invention, what is claimed is:
 1. A sinteredaluminum nitride body product comprising a substrate of an aluminumnitride having a microstructure containing a compound of aluminum oxideand calcium oxide with a layer thereover comprising a mixture of arefractory metal, a binder, and tricalcium aluminate, and a depletionzone near a surface of said body substantially free of second phaseparticles, wherein said layer is adapted to reduce formation of saidsecond phase particles in surface and subsurface microstructures thereofby forming said depletion zone when applied thereover a greensheet andsintered.
 2. The aluminum nitride body product of claim 1 wherein saiddepletion zone comprises a distance of at least about 10 to 200 micronsfrom said surface of said sintered aluminum nitride body.
 3. A sinteredaluminum nitride body product having a depletion zone for preventing theformation of particles in surface and subsurface microstructures ofaluminum nitride bodies made by the process of: (a) providing agreensheet comprising aluminum nitride powder, a sintering aid and abinder; (b) providing a paste over said greensheet comprising arefractory metal powder, a binder and a compound selected from the groupconsisting of 3CaO.Al 2 O 6 , Ca 3 Al 2 O 6 , Ca 12 Al 14 O 33 , CaAl 4O 7 , and CaAl 12 O 19 in an amount sufficient to reduce formation ofparticles in surface and subsurface microstructures of aluminum nitridebodies when applied on said greensheet and sintered; (c) applying saidpaste to at least a portion of a surface of said greensheet; and (d)heating said greensheet and paste for a time and temperature sufficientto form a sintered greensheet and paste such that a depletion zonesubstantially free of said particles having a thickness of about 10 toabout 200 microns deep from a surface of said sintered greensheet andpaste is formed.
 4. The product of claim 3 wherein said compound isselected from the group consisting of Ca 3 Al 2 O 6 , Ca 12 Al 14 O 33 ,CaAl 4 O 7 , and CaAl 12 O 19 and is present in an amount of about 5 toabout 65 volume percent.
 5. The product of claim 3 wherein said paste isapplied to said greensheet in at least two layers.
 6. The product ofclaim 3 wherein said compound is tricalcium aluminate present in anamount of about 1 to about 35 volume percent.
 7. A sintered aluminumnitride body comprising: a substrate of a sintered aluminum nitridehaving a microstructure containing a compound of aluminum oxide andcalcium oxide; a paste layer of a sintered mixture of a refractorymetal, a binder, and a compound selected from the group consisting of3CaO-Al 2 O 3 , Ca 3 Al 2 O 6 , Ca 12 Al 14 O 33 , CaAl 4 O 7 , and CaAl12 O 19 , said paste layer adapted to form a depletion zone whendisposed on a portion of said substrate in an amount sufficient toreduce formation of a precipitated yttrium aluminate compound in asurface and subsurface of said microstructure except in said depletionzone of said substrate when applied on said greensheet and sintered; andsaid depletion zone substantially free of said precipitated yttriumaluminate in said surface and subsurface of said microstructure.
 8. Thealuminum nitride sintered body of claim 7 wherein the depletion zone ofsaid substrate is depleted of the yttrium aluminate compound for a depthof about 10 to about 200 microns from said layer.
 9. The product ofclaim 3 wherein said greensheet and paste are heated for 24 hours. 10.The product of claim 3 further including in said paste an aluminumnitride powder.
 11. The product of claim 3 wherein said paste is appliedto said greensheet in a first layer and a second layer of paste, whereinsaid second layer of paste has a refractory metal powder concentrationgreater than that of the first layer of paste.
 12. A sintered aluminumnitride body product having a depletion zone substantially free ofparticles in surface and subsurface microstructures of aluminum nitridebodies made by the process of: (a) providing a greensheet comprisingaluminum nitride powder, a sintering aid and a binder; (b) providing apaste comprising a refractory metal powder, a binder, a compoundselected from the group consisting of Ca 12 Al 14 O 23 , CaAl 4 O 7 ,and CaAl 12 O 19 in an amount sufficient to reduce formation ofparticles in surface and subsurface microstructures of aluminum nitridebodies when applied on said greensheet and sintered, and about 1 toabout 50 volume percent of aluminum nitride powder; (c) applying saidpaste to at least a portion of a surface of said greensheet; (d) heatingsaid greensheet and paste for a time and temperature sufficient to forma sintered greensheet and paste; and (e) forming a depletion zonesubstantially free of said particles having a thickness of about 10 toabout 200 microns deep from a surface of said sintered greensheet andpaste.
 13. A sintered aluminum nitride body product having a depletionzone substantially free of second phase particles in surface andsubsurface microstructures of aluminum nitride bodies made by theprocess of: (a) providing a greensheet comprising aluminum nitridepowder, a sintering aid and a binder; (b) providing a paste comprising arefractory metal powder, a binder, tricalcium aluminate in an amountsufficient to reduce formation of particles in surface and subsurfacemicrostructures of aluminum nitride bodies when applied on saidgreensheet and sintered, and about 1 to about 50 volume percent ofaluminum nitride powder; (c) applying said paste to at least a portionof a surface of said greensheet; (d) heating said greensheet and pastefor a time and temperature sufficient to form a sintered greensheet andpaste; and (e) forming a depletion zone having a thickness of about 10to about 200 microns deep from a surface of said sintered greensheet andpaste.